1. Field of the Invention
The invention relates to an apparatus for the measurement of the stress birefringence of an optical sensor that is influenced by a quantity to be measured, comprising:
an optical emitting device for emitting light of two adjacent wavelengths .lambda..sub.1 and .lambda..sub.2, which device is optically connected to the sensor via an emitting light guide, a polariser and a retarder plate;
an analyser for the separation of the polarisation components of the light emerging from the sensor: and
a receiving and evaluating device, which is optically connected to the analyser via a receiving light guide and which serves to derive from the measured-value-dependent intensities of the polarisation components information concerning the magnitude of the measured value.
2. Description of the Related Art
In an apparatus of this type known from West German patent DE-A-3,138,061 the retarder plate is a .lambda./4 plate, which circularly polarises the supplied, linearly polarised light. In the sensor, the orthogonal polarisation components are then influenced in opposite senses by the measured quantity. The displacement of the operating range, caused by the .lambda./4 plate, gives a higher sensitivity of measurement. In the known case, one polarisation component of the light of one wavelength and the polarisation component orthogonal thereto of the light of the other wavelength are evaluated. Only these two components, which leave the analyser in different directions and temporally displaced, are selected by appropriate filters and passed via a coupler to a commmon light guide leading to the receiving and evaluating device.
The required filters and couplers are costly components.